Project Summary/Abstract Severe fever with thrombocytopenia syndrome virus (SFTSV) listed in the World Health Organization 9 most dangerous pathogens is an emerging Phlebovirus in the Phenuiviridae family. Due to the lack of therapy and vaccine against SFTSV infection, there is a pressing need to understand the pathogenesis of SFTSV to develop effective antiviral agents. Designated as Biosafety level 3 agent, SFTSV contains a genome comprised of three segments of negative or ambisense RNA designated as large, medium, and small. The S segment encodes a nucleoprotein and a nonstructural protein (NSs) via an ambisense coding strategy. (Aim 1) We have discovered that NSs targets the Vps15-Vps34 lipid kinase complex to form virus-induced autophagosome-like inclusion bodies (IB), subsequently sequestrating IFN signaling effectors (RIG-I, TRIM25, and TBK1) to the IB and thereby suppressing IFN production. (Aim 2) We have also found that SFTSV NSs plays an essential role in viral immunopathogenesis by targeting the TPL2-ABIN2-p105 kinase complex to robustly induce expression of immune suppressive genes, specifically IL-10 cytokine. (Aim 3) We combined viral reverse genetics, a TPL2 kinase inhibitor, Tpl2-/- and il10-/- mouse models to show that the NSs-mediated activation of TPL2 signaling pathway robustly induced IL-10 production that was essential for viral pathogenesis. For the first time, we developed an age- dependent ferret model: young adult ferrets (<2 years old) did not show any clinical symptoms and mortality; however, SFTSV-infected aged ferrets (>4 years old) demonstrated severe thrombocytopenia, reduced white blood cells, and high fever with ~90% mortality rate, fully recapitulating human clinical manifestation. With well-established in vitro experimental conditions and novel in vivo animal models, the proposed study not only demonstrates the critical role of SFTSV NSs in viral immune evasion and pathogenesis, but also identifies potential therapeutic approaches to treat SFTSV-infected patients.